RESUMENES DE COMUNICACIONES
ISBN: 978-987-544-907-7 Universidad Nacional de La Plata
FIG. 4.8 Variation in the elastic peak height round the crystal for three different incident beam energies.
source will vary with the field intensity i.e. the e^itation of the internal source depends on the incident beam geometry as determined by the Brillouin zone diagram. One® excited, the source will emit elastic and inelastic secondary electrons. Thus the variation of the total secondary electron emission is deter mined by the intensity variation of the Kikuchi pattern. Lffects caused
by variation of the incident beam direction were first seen with an 17
electron microscope by D.G.Coates and are so often referred to as Coates-Kikuchi patterns (or electron bpam channelling patterns),
Where the Kikuchi features are closely spaced the total second-
18
ary electron emission is found to be sharply attenuated . This is because the intersection of several Kikuchi lines corresponds to the exaltation of simultaneous diffracted beams which at moder ately high energies (several hundred eV or above) are in the forward direction resulting' in a greater penetration of the beam into the bulk. This results in a reduced escape probability for the secondary electrons.
Above several hundred eV poles appear in the Brillouin zone diagram1^ corresponding to directions in which no Brillouin zone boundary exists. The position of the poles is independent of the electron energy and corresponds to some low-index direction in the crystal. This can be observed in fig.
4
.6 where there is a maximum corresponding at all energies to the (110)face. It can be seen that although the position of the (110)maximum remains constant, the maxima on either side change their angular position as the beam energy changes, (the maxima are at approximately 20°,22°, and25° from the (110) direction at 1600,1200 and 1800eV respect ively).We have already seen that the Auger signal /aries in approx imately the same manner as the secondary electron current (fig.4»7)»
C
Evidently these large variations in Auger electron emission as a function of electron incidence direction are potentially dist- urbinr to temps to quantify surface compositions by
A.F.F.;
on the other hand the clean surface of an element is not a relevant test of this problem for in the absence of other species the
composition is not in doubt and quantitative studies in A.E.S. do not rely on absolute yields. Moreover, as the effects have been shown to be associated with the relatively high incident electron energies and the features correlate with bulk crystal directions,we might suppose that these angular effects would be much less important for the Auger electron yield from a species localised in, or on, the surface, because elastic backscattering is low at these energies. Evidence providing some support for this view has been provided by C h a n g é who noted that while strong angular effects of the type we show here were seen for Auger electron emission from a Si ( 111 ) substrate, the Auger peaks from a 10$ amorphous oxide on this surface showed no observable angular dépendance. However, in figure 4-9 the orientation dependence of the oxygen K W Auger electron emission at 520eV from thé copper surface after high oxygen exposure (1G,000L) is compared with the Cu substrate low energy M , W emission from both clean and oxygen
j
saturated surfaces. At lower exposures of oxygen very much larger variations in the o x y g e n Auger electron yield were seen which
are clearly attributable to real coverage variations with surface orientation; these coverage variations are the information that is sought in this investigation, the diffraction effects constitut
ing an unwanted signal. At saturation coverage the remaining angular variations appear to show the same structure as the substrate Auger electron emission indicating that the actual oxygen coverage at these exposures is essentially independant of orientation. Nevertheless, the data of fig.4.9 do show angular variations
somewhat larger than the low energy copper Anger emission for this oxyGen-covered surface. We see, therefore, that the anisot ropy is not restricted to subsurface or "bulk" effects. Pf course, at very high oxygen exposures we cannot exclude the possibility that not all the oxygen is absorbed above or within the top copper atom layer of the surface; indeed for the (100) surface there is evidence that at saturation coverage some oxygen incorporation does occur ’ but this seems to be a marginal effect at room
I
temperature a nd low oxygen pressures. X-ray photoelectron spect roscopy studies indicate the need for high partial pressures of
21
several torr to produce true CuO oxide formation.
While a large anisotropy of an adsorbate emission seems surpris ing, it appears to be in keeping with the systematic behaviour of all aspects of the secondary emission spectrum if we remember that the adsorbate species Auger emission in this case occurs at a higher energy than that of the substrate we have chosen for comparison. Fig.4.7 shows that when compa ring the twc Cu Auger emission peaks and the elasticly scattered peak the anisotropy decreases with decreasing energy. Moreover, this trend is seen in the over all background secondary electron emission itself as seen in fig.4.10 which shows selected energies of emission from the clean surface using 1500eV incident electrons. The vertical bars in each graph correspond to a 10% variation and thus aid comparisons of the different scales. This trend for the secondary electrons is not unexpected as the anisotropy appears to result from elastic scattering effects in the primary beam, and the lower the emitted energy, the greater the energy losses and directional scrambling
22
that can be expected. This is consistent with the observation that - Kikuchi-Coates patterns are most easily observed (have the greatest contrast) when only high-energ^ back-scattered electrons are collected, note that the Auger emissions ( fig 4.7) show stronger anipotrooy than the bodroround